Members of phylum Platyhelminthes have a flattened, bilaterally symmetrical body. They have three embryonic layers (triploblastic): the endoderm, the mesoderm, and the ectoderm. Flat worms have no other body cavity than the digestive cavity (acoelomates).
Members of the class Turbellaria have bodies covered with cilia and are free living. Members of the class Trematoda are parasites with a thick cuticle instead of cilia. Members of the class Cestoda are also parasites with a thick cuticle.
Parasitic flat worms have a thick cuticle for protection. Free living flat worms rely on camouflage.
Turbellians use cilia to move while parasitic worms rely on the host's circulation.
Feeding & Digestion
Flat worms have one digestive opening that branches to all parts of the body. Turbellarians have a muscular pharynx to up food. Parasitic classes rely on the host for digestion.
Circulation occurs through diffusion.
Excretion occurs through flame cells.
Respiration occurs through diffusion.
Platyhelminthes have a simple brain called a ganglion and a ventral nerve cord.
Asexual reproduction occurs through fragmentation or fission. Since the flat worms are hermaphrodites, sexual reproduction occurs through the exchange of sperm.